it's the gate mechanism inside the t350. if you tear apart the gear casing you'll notice the mechanism above the syncros and between the shift forks. get's worn and loose from trying to force the tranny into 1st at to high of a speed. rattles like a mofo until thermodyanmics takes over and the parts heat swell and fit better.

the Royal Purple 90w synthetic oil seems to stop the rattle for the most part in my tranny. try that and see what happens.

ttyl,